It is a current tendency to utilize pigments as colorants not only in various printing inks and paints but also in those fields in which clearer colorations are required, for example in color filters and inks for ink jet printing.
In recent years, marked advances have been made in the technology of dispersing such pigments. The method which is used for further rationalization of production processes comprises carrying out dispersion so as to increase the pigment concentration as far as possible. However, there is a fundamental problem that as the pigment concentration increases, the fluidity and dispersion stability decrease.
In addition, in the fields of color filters and ink jet recording, where high levels of fastness are required, it is necessary to use high-grade pigments and reduce the particle diameter to a finer level so that clearer colorations may be accomplished. However, high-grade pigments are by nature poor in dispersion stability as compared with other pigments and, further, as the particle diameter becomes finer, the tendency toward aggregation increases and it becomes difficult to obtain stable dispersions.
On the other hand, carbon black is widely used as a pigment showing a high level of blackness. However, it has problems, namely difficulty in attaining high concentrations thereof and insufficiency in dispersion stability due to its characteristics that its primary particle diameter is very small and it is porous and has a large specific surface area. Recently, attempts have been made to use carbon black also as a light-shielding material for black matrices. It is a problem, however, that it is difficult to obtain highly insulating black matrices from carbon black which is highly conductive.
Therefore, for solving such problems, investigations have so far been made concerning, for example, the treatment (surface treatment/modification) of pigments themselves or the development of pigment dispersants, pigment derivatives or surfactants, each having marked ability to wet (adsorption ability) pigments and/or be wetted to increase the pigment dispersion stability and/or pigment concentration, and a number of research works are now still going on.
For example, Japanese Kokai Publication Sho-54-37082 discloses a dispersant which comprises a reaction product derived from poly(lower alkylene)imine and a free carboxyl group-containing polyester in which at least two polyester chains are bonded to each poly(lower alkylene)imine chain; Japanese Kokai Publication Hei-07-207012 discloses a pigment dispersant produced by reacting a carboxyl group-terminated polylactone compound obtained by the ring-polymerization of a lactone compound using hydroxycarboxylic acid as an initiator with a polyalkylene polyamine compound; Japanese Kokai Publication Hei-09-169821 discloses a polyallylamine derivative introduced with a polyester, polyamide, etc. into an amino group of the polyallylamine; and further Japanese Kokai Publication Hei-09-166869 discloses a grafting method which comprises reacting a dispersant having a functional group such as an aziridine group, isocyanate group and epoxy group with a carboxyl group in carbon black having a functional group such as a carboxyl group, however, even with this method, improvements in fluidity and dispersion stability at high concentrations have not been achieved.
Thus, the applicant paid attention to a carbodiimide group having high reactivity particularly with a carboxyl group as a technology of improving the dispersibility by grafting a pigment such as carbon black, and has already developed and proposed a treated pigment capable of stably maintaining a finely dispersed state by grafting a dispersant containing the carbodiimide group within the molecule onto the surface of the above pigment (Japanese Kokai Publication 2002-201381). Furthermore, the applicant has found that the dispersion stability and fluidity of a treated pigment can be further improved by introducing a specific side chain into the molecule of a carbodiimide compound (Japanese Patent Applications No. 2002-064416, No. 2002-74966, No. 2002-89231, and No. 2002-186838).
Such a treated pigment as proposed by the applicant is utilized in printing inks, paints, ink jet recording liquids and the like and, in addition, suitably utilized particularly in resist compositions for said color filters and black matrices.
Furthermore, in recent years, dealing with high concentration of pigments which has been required, dispersing stability and fluidity become exceedingly preferable even in the case of highly-concentrated pigments, clear and high contrast coatings can be obtained in organic pigments, coatings having high insulating property and light-shielding property can be obtained in carbon black, further developing ability is improved when used as a resist composition; and other excellent effects are to be obtained.
However, the above-mentioned technology is highly effective for pigments having a functional group reactive with a carbodiimide group on the surfaces, but there has been a problem that the effect is hardly obtained for those pigments having no such functional group. Further, when the pigment is used as a resist composition, higher developability is sometimes required, and there has been room for further improvements.